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Date:   Mon, 12 Dec 2022 11:12:56 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Douglas Raillard <douglas.raillard@arm.com>
Cc:     linux-kernel@vger.kernel.org,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Valentin Schneider <vschneid@redhat.com>
Subject: Re: [for-next][PATCH 02/11] tracing: Add __cpumask to denote a
 trace event field that is a cpumask_t
Message-ID: <20221212111256.3cf68f3e@gandalf.local.home>
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On Mon, 12 Dec 2022 14:53:27 +0000
Douglas Raillard <douglas.raillard@arm.com> wrote:

> On 24-11-2022 14:50, Steven Rostedt wrote:
> > From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> > 
> > The trace events have a __bitmask field that can be used for anything
> > that requires bitmasks. Although currently it is only used for CPU
> > masks, it could be used in the future for any type of bitmasks.
> > 
> > There is some user space tooling that wants to know if a field is a CPU
> > mask and not just some random unsigned long bitmask. Introduce
> > "__cpumask()" helper functions that work the same as the current
> > __bitmask() helpers but displays in the format file:
> > 
> >    field:__data_loc cpumask_t *[] mask;    offset:36;      size:4; signed:0;

The current parsing tools break the above into:

 "field:" "__data_loc" <some-type> "[]" <var-name> ";" "offset:"
 <offset> ";" "size:" "<size>" ";" "signed:" <signed> ";"

Where the <some-type> really can be anything, and in lots of cases, it is.
Thus its only a hint for the tooling, and has never been limited to what
they are.

> > 
> > Instead of:
> > 
> >    field:__data_loc unsigned long[] mask;  offset:32;      size:4; signed:0;
> > 
> > The main difference is the type. Instead of "unsigned long" it is
> > "cpumask_t *". Note, this type field needs to be a real type in the
> > __dynamic_array() logic that both __cpumask and__bitmask use, but the
> > comparison field requires it to be a scalar type whereas cpumask_t is a
> > structure (non-scalar). But everything works when making it a pointer.  

The above is for the kernel to build.

> 
> How is tooling expected to distinguish between a real dynamic array of pointers
> from a type that is using dynamic arrays as an "implementation detail"
> with a broken type description ? Any reasonable
> interpretation of that type by the consuming tool will be broken
> unless it specifically knows about __data_loc cpumask*[].

I'm curious to what the tool does differently with the above. What tool are
you using? Does it just give up on how to print it?

> However, the set of types using that trick is unbounded so forward
> compatibilty is impossible to ensure. On top of that, an actual
> dynamic array of cpumask pointers becomes impossible to represent.

I never thought about a user case where we print out an array of cpumask
pointers.

> 
> You might object that if the tool does not know about cpumask,
> it does not matter "how it breaks" as the display will be useless anyway,
> but that is not true. A parsing library might just parse up to
> its knowledge limit and return the most elaborate it can for a given field.
> It's acceptable for that representation to not be elaborated with the full
> semantic expected by the end user, but it should not return
> something that is lying on its nature. For example, it would be sane for
> the user to assert the size of an array of pointers to be a multiple
> of a pointer size. cpumask is currently an array of unsigned long but there is
> nothing preventing a similar type to be based on an array of u8.
> Such a type would also have different endianness handling and the resulting buffer
> would be garbage.
> 
> 
> To fix that issue, I propose to expose the following to userspace:
> 1. The binary representation type (unsigned long[] in cpumask case).
> 2. An (ordered list of) semantic type that may or may not be the same as 1.
> 
> Type (1) can be used to guarantee correct handling of endianness and a reasonable
> default display, while (2) allows any sort of fancy interpretation, all that while preserving
> forward compatibility. For cpumask, this would give:
> 1. unsigned long []
> 2. bitmask, cpumask
> 
> A consumer could know about bitmask as they are likely used in multiple places,
> but not about cpumask specifically (e.g. assuming cpumask is a type recently introduced).
> Displaying as a list of bits set in the mask would already allow proper formatting, and
> knowing it's actually a cpumask can allow fancier behaviors.
> 
>  From an event format perspective, this could preserve reasonable backward compat
> by simply adding another property:
> 
>    field:__data_loc unsigned long[] mask;    offset:36;      size:4; signed:0; semantic_type:bitmask,cpumask;
> 
> By default, "semantic_type" would simply have the same value as the normal type.

The problem with the above is that it adds a new field, and I have to check
if that doesn't break existing tooling.

Another possibility is that I can add parsing to the format that is exposed
to user space and simply s/__cpumask *[]/__cpumask[]/

Which will get rid of the pointer array of cpu masks.

> 
> This applies to any type, not just dynamic arrays.
> 

Let me know if the above does break existing user space and I'll revert it.

-- Steve